A photoconductive steps carried out by an electrophotographic light-sensitive material are comprised of:
(1) generating a charge by light exposure; and
(2) transporting the charge.
As an example of performing steps (1) and (2) with the same material, there is illustrated a selenium light-sensitive plate. On the other hand, as an example of performing steps (1) and (2) with different materials, there is the well-known combination of amorphous selenium and poly-N-vinylcarbazole. The process of performing the steps (1) and (2) by different or separate materials is disirable because the range of selecting materials used for the light-sensitive materials can be enlarged, thereby the electrophotographic characteristics of the light-sensitive materials, such as sensitivity, allowable electric potential, etc., are improved, and also materials suitable for forming coated layers of the light-sensitive materials can be selected over a wide range.
Hitherto, inorganic materials such as selenium, cadmium sulfide, zinc oxide, etc., have been used as the materials for photoconductive materials of light-sensitive materials used in an electrophotographic system.
As disclosed by Carlson in U.S. Pat. No. 2,297,691, in an electrophotographic process a photoconductive material comprising a support coated with a material which is insulating in the dark but changes its electric resistance according to the exposure amount irradiated during image exposure is used. The photoconductive material is generally subjected to a dark adaptation for a proper period of time and then the surface thereof is first uniformly charged in the dark. Then, the material is image-exposed by irradiation patterns having an effect of reducing the surface charge according to the corresponding energy contained in various portions of the irradiated patterns. The surface charge or an electrostatic latent image thus remaining on the surface of the photoconductive material layer (light-sensitive layer) is brought into contact with a proper developer, i.e., a toner to provide a visible image. The toner which is contained in a insulating liquid or in a dry carrier can be attached to the surface of the light-sensitive layer in conformity to the charge patterns. The toner thus attached can be fixed by a known means such as heat, pressure, solvent vapor, etc. Also, the lectrostatic latent image can be transferred onto a 2nd support (e.g., a paper, a film, etc.,) and the electrostatic latent image thus tranferred onto the 2nd support can be developed there by the same manner as above. The electrophotographic process is one of the image-forming processes for forming images by such a manner.
The fundamental characteristics required for the light-sensitive material in such an electrophotographic process are (1) it can be charged at a proper potential in the dark, (2) the charge on the surface of it is reluctant to dissipate in the dark, and (3) the charge can be quickly dissipated by the irradiation of light. It is true that the foregoing inorganic materials conventionally used have many merits and at the same time various demerits. For example, selenium which is widely used at present sufficiently meets the foregoing factors (1)-(3) but is not desirable because the conditions for producing selenium layer are troublesome which causes an increase in the production cost thereof, it is difficult to form a belt-like light-sensitive material composed of selenium since it has no plasticity, and it must be handled carefully since it is sensitive to heat and mechanical impact. Cadmium sulfide and zinc oxide are used as a light-sensitive material in a form of a dispersion in a binder of a resin. However, such a photoconductive layer has physical faults with respect to its smoothness, hardness, tensile strength, abrasion resistance. Accordingly, it cannot be repeatedly used.
Recently, in order to eliminate the faults of the inorganic materials, electrophotographic light-sensitive materials using various organic materials have been proposed and practically used. For example, there have been proposed a light-sensitive material composed of poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (see, e.g., U.S. Pat. No. 3,484,237), a light-sensitive material composed of poly-N-vinylcarbazole sensitized by a pyrylium salt series dye (see, e.g., U.S. Pat. No. 3,617,268), a light-sensitive material mainly composed of an organic pigment (see, e.g., U.S. Pat. No. 3,898,084), and a light-sensitive material mainly composed of an eutectic complex of a dye and a resin (see, e.g., U.S. Pat. Nos. 3,732,180 and 3,684,502). These light-sensitive materials have excellent characteristics and are considered to have high practical value. However, it is necessary to consider various requirements for the light-sensitive materials such as: the simplicity of the production process, the electrophotographic characteristics, and the wave length selectivity (required when applying the electrophotographic light-sensitive materials to a laser beam printer or an indication element). Materials sufficiently meeting these requirements have not yet been obtained at present.